Distribution of the HCO⁺/N₂H⁺ abundance ratios towards our source sample (see Table 1). The sources are sorted by increasing luminosities. Full dots with error bars indicate the abundance ratios that have been corrected for the line opacity when H¹³CO⁺ (6–5) observations are available (see Sects. 4.1 and 4.2). Red circle marks the colder source of our sample (based on the HCO⁺ SLED, see Sect. 3.2). The blue filled square shows the HCO⁺/N₂H⁺ ratio reported in direction of OMC-2 FIR4 by Ceccarelli et al. (2014). Finally, the red dashed lines show the predicted ratios that are expected for cosmic-rate ionization rates, ζ, of 10^-14 s^-1, 10^-15 s^-1 and 3 × 10^-17 s^-1, for a gas temperature of 40 K and a n_H2 density of 2.5 × 10⁵ cm^-3 (for further details see Ceccarelli et al. 2014).